Add support for .sqlfluff

Documented as `ini` here: https://github.com/sqlfluff/sqlfluff/blob/5d5fe0a6d3fca6c49c36c3d35af6952984f0bb8f/docs/source/configuration/setting_configuration.rst#L29-L33

Author: mxr

identify/extensions.py

@@ -352,6 +352,7 @@
     '.pypirc': EXTENSIONS['ini'] | {'pypirc'},
     '.rstcheck.cfg': EXTENSIONS['ini'],
     '.salt-lint': EXTENSIONS['yaml'] | {'salt-lint'},
+    '.sqlfluff': EXTENSIONS['ini'],
     '.yamllint': EXTENSIONS['yaml'] | {'yamllint'},
     '.zlogin': EXTENSIONS['zsh'],
     '.zlogout': EXTENSIONS['zsh'],